The background description provided herein is for the purpose of generally presenting the context of the disclosure and may not constitute prior art.
Diesel engines have been used in a variety of applications such as locomotives, marines and engine-generators. The U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (GARB), as well as other regulatory agencies around the world, impose strict limitations on the contents of emissions from diesel engines, such as particulate matter (PM), hydrocarbon (HC) and NOx. Accordingly, exhaust aftertreatment systems have been employed and generally include a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), and an SCR (Selective Catalytic Reduction of NOx) to treat the exhaust gas and to control emissions to atmosphere or the outside environment.
Various chemical reactions occur in the DOC and SCR to convert harmful nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbon (HC) into N2, CO2 and water. The DPF is designed to remove diesel particulate matter (PM) from the exhaust gas. Normally these chemical reactions would take place at high temperatures. With the use of catalysts, the chemical reactions can occur at much lower temperatures. Sufficient energy in the form of heat, however, must still be supplied to the catalysts to expedite the chemical reactions. Therefore, performance of the exhaust aftertreatment system is highly dependent on the temperature of the exhaust gas, which carries the desired energy and heat to the catalysts. The normal temperature of the exhaust gas, however, does not always meet requirements for the desired chemical reactions. When the normal exhaust temperature is lower than the target temperature, the exhaust aftertreatment system cannot effectively treat the exhaust gas, resulting in higher emissions to the outside environment.
One method of increasing the exhaust gas temperature is through injecting hydrocarbon upstream from a DOC either in the exhaust pipe or inside the cylinder during the exhaust stroke. This method increases fuel consumption and also changes composition of the exhaust gas. For example, when fuel injection is injected in the exhaust, NO2 generation in the DOC is significantly reduced. NO2 is an effective reagent for passive regeneration of DPF at much lower temperature range. Therefore, the reduced NO2 generation adversely affects the passive regeneration of the DPF.